1. Field of the Invention
This invention relates generally to free piston Stirling engines, free piston Stirling heat pumps or coolers, compressors or other mechanical loads drivingly connected to a linear alternator or linear motor and more particularly relates to a circuit for sensing, and for providing a signal that is proportional to, the instantaneous position of the reciprocating piston of the Stirling engine, cooler or heat pump or compressor and the instantaneous position of the reciprocating member of the linear alternator or motor.
2. Description of the Related Art
Stirling machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because of advantages they offer. In a Stirling machine, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Stirling machines cyclically shuttle a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, regenerator and heat rejecter. The shuttling is commonly done by pistons reciprocating in cylinders and cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and/or gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and/or gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in both spaces at any instant of time because the spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm sink.
Stirling machines can therefore be designed to use the above principles to provide either (1) an engine having pistons driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space, or (2) a heat pump having pistons cyclically driven by a prime mover for pumping heat from the expansion space to the compression space. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the pistons are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. Free-piston Stirling machines are constructed as mechanical oscillators and one of its pistons, conventionally identified as a displacer, is driven by the working gas pressure variations in the machine. The other piston, conventionally identified as the power piston, is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load when the Stirling machine is operated as an engine. Free piston Stirling machines offer numerous advantages including the control of their frequency and phase and their lack of a requirement for a seal between moving parts to prevent the mixing of working gas and lubricating oil.
Stirling machines are often connected to a linear motor or alternator. A Stirling engine is connected to a linear alternator to generate electric power and a Stirling machine operated in a heat pumping mode is connected to and driven by a linear electric motor. Both an electric linear motor and an electric linear alternator are the same basic device. They have a stator, ordinarily having an armature winding, and a reciprocating member that includes magnets, usually permanent magnets. Because they are essentially the same fundamental structure, they will sometimes be collectively referred to as a motor/alternator.
Consequently, both a Stirling machine and a linear motor/alternator structure are energy transducers that can each be operated in either of two modes. A Stirling machine can be driven mechanically in reciprocation by a prime mover to pump heat from a lower temperature mass to a higher temperature mass. A Stirling machine can be driven by the energy of the temperature difference between two masses and provide an output of mechanical reciprocation. Similarly, a linear motor/alternator structure can be mechanically driven in reciprocation by a prime mover to generate electrical power output or a linear motor/alternator be driven by a source of alternating electrical power to operate as a motor providing a mechanical reciprocating output. Therefore, a Stirling machine operating as an engine can be used to drive a linear alternator and a linear motor can be used to drive a Stirling machine operating in a heat pumping mode. In both of these cases, the power piston of the Stirling machine is ordinarily directly connected to the reciprocating member of the linear motor or alternator so that they reciprocate as a unit. Additionally, linear electric motors can be connected to and used to drive the piston of a compressor for compressing a gas, such as in a refrigerator, or for pumping a fluid.
All of these combinations utilize control circuits not only to assure that the pistons do not reciprocate at an excessive stroke and collide with other machine structures, but also to maximize their operating efficiency, control power or control other operating parameters. Therefore, one important parameter of operation that is useful to detect or sense and apply a corresponding signal to the control circuit for controlling these machines is the instantaneous position of the piston and its connected reciprocating member of the linear motor or alternator.
One device that has been proposed for sensing the piston position is illustrated in U.S. Pat. No. 4,667,158, which is herein incorporated by reference. However, that invention requires that a special transducer, that is dedicated to sensing the piston position, be mounted in the machine. This transducer has a ferromagnetic tube, a coil and a core all of which must be located and mounted within the machine. That transducer has the disadvantage that it adds weight and additional expense, requires mounting and occupies space within the machine and is an additional component that can fail.
Another proposed manner of detecting piston position is described in U.S. Pat. No. 5,342,176 which is herein incorporated by reference. The invention of that patent computes piston position from the back emf induced on the windings by the magnets and the current of the electric linear motor at the operating frequency of the linear motor. This prior art detection system has the disadvantage that it requires measurement of the inductance of the alternator winding, the AC resistance and the back emf constant (also known as the motor constant) α. The open circuit winding voltage is proportional to the piston velocity and the back emf constant α is the proportionality constant that relates open circuit winding voltage to piston velocity. The winding current is directly proportional to the force on the piston and the proportionality constant is also α. The back emf constant α is temperature dependent as is the winding resistance. Consequently, measurements of stroke amplitude using the method of U.S. Pat. No. 5,342,176 exhibit errors resulting from drift of the detected stroke amplitude as a function of temperature.
It is therefore an object and feature of the invention to provide a circuit for detecting the instantaneous position of the reciprocating member and a piston connected to it that does not require the installation of additional hardware within the machine and provides a signal that is proportional to the instantaneous position of the piston and its connected reciprocating member of an electric linear motor/alternator structure but also is independent of the voltage and current at the operating frequency of reciprocation of the machine.